Thermal recording sheet

ABSTRACT

In a thermal recording sheet having a thermal recording layer containing a colorless or pale colored dye precursor and a color developer reactable with the dye precursor upon heating to develop a color, a dimerized or trimerized urea compound is used as the color developer to obtain a thermal recording sheet which is superior in ground color stability and a thermal recording sheet having a reversible recordability.

FIELD OF THE INVENTION

This invention relates to a thermal recording sheet with improvedstability of the ground color and to a thermal recording sheet having areversible recording ability with a bright color tone.

DESCRIPTION OF THE PRIOR ART

In general, to obtain a thermal recording sheet, a colorless or palecolored electron-donating dye and a color developer such as phenoliccompounds are individually pulverized to fine particles, mixed with eachother, and a binder, a filler, a sensitizer, a slip agent, and otheradditives are added to obtain a coating color, which is coated on asubstrate such as paper, synthetic paper, films, plastics, or the like.The coated sheet is color developed by an instantaneous chemicalreaction by heating with a thermal head, a hot stamp, laser light, orthe like to obtain a visible record.

The thermal recording sheet is used in a wide variety of applicationssuch as measuring recorders, a terminal printer for computers,facsimile, automatic ticket vendors, bar-code labels, and the like.However, with recent diversification and advance in recording apparatusfor thermal recording sheet, the thermal recording sheet is required tomeet increased requirements. For example, with increasing speed ofrecording, it is required to obtain a high-density and sharp color imageeven with a smaller thermal energy, whereas, in view of storagestability of the recording sheet, it is required to have improved lightresistance, oil resistance, water resistance, and solvent resistance.

On the other hand, with increasing use of systems for recording on plainpaper such as electrophotographic and ink jet recording systems, thermalrecording becomes often compared with such plain paper recording. Forexample, thermal recording is required to be close in quality to plainpaper recording in terms of stability of recorded portion (image) andstability of unrecorded portion (ground color portion) before and afterrecording (hereinafter referred to as ground color stability). Inparticular, as a basic function of thermal recording, the sheet isrequired to have a good ground color stability to heat and solvents,that is, the sheet is desired to be heat-sensitive only duringrecording, but non-sensitive in other times.

For the ground color stability of thermal recording sheet, for example,Japanese Patent Laid-open Publication (OPI) 4-353490 discloses a thermalrecording sheet which undergoes no deterioration in the whiteness ofground color even at high temperatures of about 95° C. This thermalrecording sheet contains 4-hydroxydiphenylsulfone compound and the metalsalt of phosphate.

Further, rapid increases in consumption of recording sheets due toincreasing construction of various networks, and use of facsimiles andcopiers have a social problem of waste treatment. As a measure ofsolving such a problem, a reversible recording material which can berepeatedly recorded and erased is drawing attention.

Reversible recording materials are described in detail in literaturessuch as the Bulletin of the Pulp and Paper Association 47 11 (1993)p1309-1322. For example, Japanese OPIs 3-230993 and 4-366682 discloserecording materials which utilize a reversible change of the recordingmaterial between a transparent state and a white turbid state. Inaddition, recording materials which use reversibility of a thermochromicsubstance or a reversible color change of a leuco dye are proposed.

The thermal recording material disclosed in Japanese OPI 4-353490 showsa Macbeth density of ground color of about 0.11 after heating in a dryerat 95° C. for 5 hours, which is a fairly good stability but stillinsufficient, in term of ground color stability at 120° C.

For the reversible recording materials, one which uses a reversiblechange of the recording material between a transparent state and aturbid state (in other words, a recording material which utilizes achange in transparency of the recording material) has such disadvantagesthat: (1) image sharpness is poor, (2) clouding (discoloration) speed islow, and (3) when erasing, temperature control is required. Since thisrecording material obtains a contrast between transparency andturbidity, there is no problem when it is used in transparent recordingsheets such as OHP sheet, but for example, when it is used in opaquerecording sheets such as facsimile paper, it is necessary to provide acoloring layer under the coating layer (the layer capable of reversiblechange between transparent and turbid atates). For this purpose, toprovide a contrast in the coating layer, that is, to develop a sharpcolor of the color layer, the coating layer is desirably a thin layeror, to make the sheet look white, a thicker layer is preferable.Therefore, the thickness of the coating layer must be strictlycontrolled.

Further, in a reversible recording sheet using a thermochromic material,most thermochromic materials are poor in memorizing properties, and thusrequire continuous supply of heat to maintain a color developingcondition.

On the other hand, as reversible recording sheets which use leuco dyesas chromogenic sources, Japanese OPIs 60-193691 and 60-257289 discloserecording sheets, but these sheets are erased with water or steam, andthus have a problem in practical applications. Japanese OPIs 2-188293and 2-188294 disclose materials (color developing/erasing agents) whichare simple in structure and have both color developing and erasingactions to endow a leuco dye with a reversible change in color by thecontrol of thermal energy.

However, these color developing/erasing agents involve an erasingprocess already in the color developing process, and are thus low incolor developing density. Further, recently, in Japan Hardcopy 1993,Yokota et al. reports a recording sheet which uses an amidophenolderivative having a long-chain alkyl group, but this recording sheetrequires temperature control during erasing.

Therefore, a primary object of the present invention is to providethermal recording sheet which has improved ground color stability toheat and solvents as basic functions. As additional functions, thepresent invention is to provide a thermal recording sheet which hasreversible recording capability enabling recording--erasing--re-recording.

SUMMARY OF THE INVENTION

The above problems are solved by a thermal recording sheet using ureacompound, which is quite different from conventional phenolic colordevelopers, as a developer.

Thermal recording sheets using urea compound with phenol type developer(Bisphenol A, Bisphenol S, and so on.) are disclosed in Japanese OPIs53-140043, 57-87993, 57-82787, and 59-67083. These urea compounds areonly limited to the carbon number, and are basically used for depressionof melting point of phenoltype developer. Thermal recording sheets usingurea compound together phenol type developer have a problem of heatresisitance, and have no recording reversibility.

Patents on monourea compounds for use in thermal recording materials aredisclosed in Japanese OPIs 58-211496, 59-184694, and 61-211085. Theseurea compounds are those in which only the amino group at one side ofurea is substituted, and thus basically different from the dimerized andtrimerized phenylurea compounds of the present invention.

In general, thermal recording materials are used by dispersing in water,but monourea compounds have a problem since they have a slightsolubility in water. Thermal recording materials using these monoureacompounds alone have a problem of heat resistance, and have noreversible recording.

The dimerized urea type compounds are disclosed in Japanese OPIs5-1317152 and 5-147357. These dimerized urea type compounds arecharacterized by a structure having sulfonyl group adjacent to the ureastructure (Ar--SO₂ --NH--C(═O)--NH--). However, the thermal recordingsheets using these dimerized urea also have a problem in heatresistance, and have no reversible recording.

In accordance with the present invention, there is provided in a thermalrecording sheet having a thermal recording layer containing a colorlessor pale colored dye precursor and a color developer reactable with thedye precursor upon heating to develop a color, characterized in that thecolor developer is a urea compound having at least two groups of Formula(1) (hereinafter referred to as "phenylurea structure") in the molecule,and the thermal recording layer contains at least one of the ureacompound. ##STR1## wherein X is alkyl group having 1 to 12 carbon atoms,halogenated alkyl group having 1 to 6 carbon atoms, aralkyl group having7 to 14 carbon atoms, alkoxy group having 1 to 12 carbon atoms, aryloxygroup having 6 to 12 carbon atoms, alkoxycarbonyl group having 1 to 12carbon atoms, acyl group having 1 to 12 carbon atoms, dialkylamino grouphaving 1 to 12 carbon atoms, arylalkylamino group having 7 to 12 carbonatoms, arylamino group having 6 to 12 carbon atoms, acylamino grouphaving 1 to 12 carbon atoms, nitro group, cyano group, halogen group, orhydrogen group.

As the urea compound used in the present invention having at least twogroups of Formula (1) in the molecule, those of Formula (2), (3), or (4)are practically preferable. In other words, it is preferable that in thethermal recording sheet having a thermal recording layer containing acolorless or pale colored dye precursor and a color developer whichreacts with the dye precursor upon heating to develop a color, the colordeveloper is a urea compound of Formula (2), (3), or (4), and thethermal recording layer contains at least one of the urea compounds.##STR2## wherein X is alkyl group having 1 to 12 carbon atoms,halogenated alkyl group having 1 to 6 carbon atoms, aralkyl group having7 to 14 carbon atoms, alkoxy group having 1 to 12 carbon atoms, aryloxygroup having 6 to 12 carbon atoms, alkoxycarbonyl group having 1 to 12carbon atoms, acyl group having 1 to 12 carbon atoms, dialkylamino grouphaving 1 to 12 carbon atoms, arylalkylamino group having 7 to 12 carbonatoms, arylamino group having 6 to 12 carbon atoms, acylamino grouphaving 1 to 12 carbon atoms, nitro group, cyano group, halogen group, orhydrogen group. A denotes a divalent group comprising 30 or less carbonatoms, and B denotes a trivalent group comprising 30 or less carbonatoms.

The urea compound of Formula (2), (3), or (4) is a compound having twoor three phenylurea structures of Formula (1). Tile urea compound ofFormula (2) is a compound in which two phenylurea structures aredirectly bonded, and the urea compound of Formula (3) or (4) is acompound in which two or three phenylurea structures are bonded througha joint group. The joint group is a divalent or trivalent groupcomprising 30 or less carbon atoms.

The urea compound of Formula (2) or (3) (hereinafter referred to as"dimerized urea compound") and the urea compound of Formula (4)(hereinafter referred to as "trimerized urea compound") can beindividually synthesized, for example, by a reaction of amines with anisocyanate compound.

    --NH.sub.2 --+--N═C═O→--NH--C(═O)--NH--

First, the dimerized urea compound of Formula (3) can be synthesized (a)by reacting diamines with a monoisocyanate compound, or (b) by reactinga diisocyanate compound with monoamines.

    H.sub.2 N--X--NH2+Y--N═C═O →Y--NH--C(═O)--H--X--NH--C(═O)--NH--Y      (a)

    O═C═N--X--N═C═O+Y--NH.sub.2 →Y--NH--C(═O)--NH--X--NH--C(═O)--NH--Y     (b)

When diamines are reacted with a monoisocyanatc compound, the diaminesinclude carbohydrazide, ethylenediamine, 1,2-propanediamine,1,3-propanediamine, 1,4-diaminobutane, 1,3-diaminopentane,1,5-diaminopentane, 1,6-diaminohexane, 1,7-diaminoheptane,1,8-diaminooctane, 1,9-diaminononane, 1,10-diaminodecane,1,11-diaminoundecane, 1,12-diaminododecane, 1,2-diamino-2-methylpropane,1,3-diamino-2,2-dimethylpropane, 1,5-diamino-2-methylpentane,1,5-diamino-2,2-dimethylpentane, trimethylhexamethylenediamine,diethylenetriamine, triethylenetetramine, tetraethylenepentamine,3,3'-diaminodipropylamine, N,N-bis(3-aminopropyl)methylamine,bis(3-aminopropyl) ether, 1,8-diamino-3,6-dioxaoctane,ethyleneglycolbis(3-aminopropyl)ether, 1,2-diaminocyclohexane,1,3-diaminocyclohexane, 1,4-diaminocyclohexane,1,3-di(aminomethyl)cyclohexane, 1,4-di(aminomethyl)cyclohexane,4,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone,4,4'-diaminodiphenylsulfide, 4,4'-diaminodiphenylether,3,4'-diaminodiphenylether, 4,4'-diaminodiphenylether,3,4'-diaminodiphenylether, 4,4'-diaminodicyclohexylmethane,4,4'-diaminodiphenylmethane, 3,3'-diaminodiphenylmethane,4,4'-diamino-3,3'-dichlorodiphenylmethane,4,4'-diamino-1,2-diphenylethane, 4,4'-diamino-2,2'-dimethylbiphenyl,4,4'-diaminodiphenylamine, 2,2'-diaminodiphenyldisulfide,4,4'-diaminodiphenyldisulfide, 1,3-bis(m-aminophenoxy)benzene,1,4-bis(m-aminophenoxy)benzene, 2,2-bis(4-aminophenxyphenyl)propane,2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane,a,a'-bis(4-aminophenyl)-1,4-diisopropylbenzen 3,3'-diaminobenzophenone,4,4'-diaminobenzophenone, 2,4-diaminoazobenzene, 4,4'-diaminostilbene,4,4'-diaminostilbene-2,2'-disulfonic acid, o-phenylenediamine,m-phenylenediamine, p-phenylenediamine, 4-chloro-o-phenyIenediamine,5-chloro-m-phenylenediamine, 2-chloro-p-phenylenediamine,2-nitro-1,4-phenylenediamine, 4-nitro-1,2-phenylenediamine,4-nitro-1,3-phenylenediamine, 2,4-diaminoanisole,p,p'-methylenedianiline, m-xylylenediamine, p-xylylenediamine,2,4-diaminotoluene, 2,6-diaminotoluene, 3,4-diaminobenzoic acid,3,5-diaminobenzoic acid, 1,5-diaminonaphthalene, 1,8-diaminonaphthalene,2,3-dianononaphthalene, 1,2-diaminoanthraquinone,1,4-diaminoanthraquinone, 1,5-diaminoanthraquinone,2,6-diaminoanthraquinone, 1,4-diamino-2,3-dicyano-9.10-anthraquinone,2,3-diaminopyridine, 2,6-diaminopyridine, 3,4-diaminopyridine,2,4-diamino-6-chloropyrimidine, 2,4-diamino-6-phenyl-1,3,5-triazine,9,9-bis(4-aminophenyl)fluorene, piperazine, N-aminoethylpiperazine,1,4-bid(3-aminopropyl)piperazine, and isophoronediamine; themonoisocyanate compound includes aromatic isocyanate compounds such asphenylisocyanate, o-tolylisocyanate, m-tolylisocyanate,p-tolylisocyanate, 3,4-dimethylphenylisocyanate,2,6-dimethylphenylisocyanate, 3,4,5-trimethyl phenylisocyanate,o-trifluoromethylphenylisocyanate, m-trifluoromethylphenylisocyanate,p-trifluoromethylphenylisocyanate, 2-methoxyphenylisocyanate,3-methoxyphenylisocyanate, 4-methoxyphenylisocyanate,p-bromophenylisocyanate, o-fluorophenylisocyanate,m-fluorophenylisocyanate, p-fluorophenylisocyanate,o-chlorophenylisocyanate, 2,5-dichlorophenylisocyanate,3,4-dichlorophenylisocyanate, 2,6-dichlorophenylisocyanate,o-nitrophenylisocyanate, m-nitrophenylisocyanate,pnitrophenylisocyanate, p-dimethylaminophenylisocyanate, andp-diethylaminophenylisocyanate, and any combinations of both compoundscan be used.

On the other hand, when a diisocyanate compound is reacted withmonoamines, the diisocyanate compound includes 2,4-toluenediisocyanate(2,4-TDI), 2,6-toluenediisocyanate (2,6-TDI),4,4'-diphenylmethanediisocyanate (MDI), 1,5-naphthalenediisocyanate(NDI), tolidinediisocyanate (TODI), hexamethylenediisocyanate (HDI),isophoronediisocyanate (IPDI), p-phenylenediisocyanate,m-phenylenediisocyanate, trans-cyclohexane-1,4-diisocyanate,xylylenediisocyanate (XDI), 4,4'-dicyclohexylmethanediisocyanate(hydrogenated MDI), lysinediisocyanate (LDI), m-tetramethylxylenediisocyanate (m-TMXDI), and p-tetramethylxylenediisocyanate (p-TMXDI);the monoamines include aromatic amines such as aniline, o-toluidine,m-toluidine, p-toluidine, o-ethylaniline, m-ethylaniline,p-ethylaniline, p-n-propylaniline, p-iso-propylaniline,p-n-butylaniline, 2-sec-butylaniline, 4-sec-butylaniline,4-tert-butylaniline, p-n-amylaniline, p-n-hexylaniline,p-n-heptylaniline, p-n-octylaniline, p-n-nonylaniline, p-n-decylaniline,p-undecylaniline, p-dodecylaniline, 2,3-xylidine, 2,4-xylidine,2,5-xylidine, 8,4-xylidine, mesitylamine, 2,6-diethylaniline,o-aminobenzotrifluoride, m-aminobenzotriluoride,p-aminobenzotrifluoride, o-methoxyaniline, m-methoxyaniline,p-methoxyaniline, o-ethoxyaniline, m-ethoxyaniline, p-ethoxyaniline,4-tert-butoxyaniline, 2,4-dimethoxyaniline, 2,5-dimethoxyaniline,3,4-dimethoxyaniline, 3,5-dimethoxyaniline, o-chloroaniline,m-chloroaniline, p-chloroaniline, o-bromoaniline, m-bromoaniline,p-bromoaniline, o-fluoroaniline, m-fluoroaniline, p-fluoroaniline,2,3-dichloroaniline, 2,4-dichloroaniline, 2,5-dichloroaniline,2,6-dichloroaniline, 3,4-dichloroaniline, 3,5-dichloroaniline,2,4-dibromoaniline, 2,5-dibromoaniline, 2,6-dibromoaniline,2,4-difluoroaniline, 2,5-difloroaniline, 2,6-difluoroaniline,3,4-difluoroaniline, 2,3,4-trichloroaniline, 2,4,5-trichloroaniline,2,4,6-trichloroaniline, o-nitroaniline, m-nitroaniline, p-nitroaniline,o-cyanoaniline, m-cyanoaniline, p-cyanoaniline, methyl o-aminobenzoate,methyl m-aminobenzoate, methyl p-aminobenzoate, ethyl o-aminobenzoate,ethyl m-aminobenzoate, ethyl p-aminobenzoate, n-propyl p-aminobenzoate,iso-propyl p-aminobenzoate, n-butyl p-aminobenzoate, iso-butylp-aminobenzoate, phenyl o-aminobenzoate, phenyl m-aminobenzoate, phenylp-aminobenzoate, benzyl o-aminobenzoate, benzyl m-aminobenzoate, benzylp-aminobenzoate, p-nitrophenyl o-aminobenzoate, 2-diethylaminoethylp-aminobenzoate, 2'-aminoacetophenone, 3'-aminoacetophenone,4'-aminoacetophenone, 2-aminobenzophenone, 3-aminobenzophenone,4-aminobenzophenone, m-aminobenzoylmethylamide, 2-aminodiphenylether,3-aminodiphenylether, 4-aminodiphenylether,2-amino-4'-chlorodiphenylether, 4-amino-4'-chlorodiphenylether,4-amino-4'-aminodiphenylmethane, 4-amino-4'-chlorodiphenylmethane,2-amino-5-chlorotoluene, 4-amino-2-chlorotoluene,2-amino-5-boromobenzotrifluoride, 2-amino-5-chlorobenzotrifluoride,3-amino-4-chlorobenzotrifluoride, 5-amino-2-chlorobenzotrifluoride, andany combinations of both compounds can be used.

The dimerized urea compound of Formula (2) can be synthesized usinghydrazine as the diamine and the above compounds as the monoisocyanatecompound. On the other hand, the trimerized urea compound of Formula (4)can be synthesized by a method of reacting triamines with amofioisocyanate compound or a method of reacting a triisocyanatecompound with monoamines.

First, when triamines are reacted with a monoisocyanate compound, thetriamines can be 1,2,4-triaminobenzene, tris(2-aminoethyl)amine,melamine, 2,4,6-triaminopyrimidine, and triamtherene, the monoisocyanatecompound can be the above described compounds, and any combinations ofboth compounds can be used. When a triisocyanate compound is reactedwith monoamines, the triisocyanate compound can betriphenylmethanetriisocyanate, tris(isocyanatephenyl)thiophosphate,lysine ester triisocyanate, 1,6,11-undecanetriisocyanate,1,8-diisocyanate-4-isocyanatemethyloctane,1,3,6-hexamethylenetriisocyanate, and bicycloheptanetriisocyanate; themonoamines can be the above described compounds, and any combinations ofboth compounds can be used.

Practical examples of the urea compound of Formula (2), (3), or (4) areshown below: ##STR3##

The joint group is to be appropriately selected according to the meltingpoint, decomposition temperature, solubility to solvents, and the likeof the urea compound, and to the property of the thermal recordingsheets using the urea compound as a developer, and then is notspecifically limited. For example, for the thermal recording sheet usingthe dimerized urea compound of Formula (3), the joint groups are broadlydivided by the joint group as follows:

(1) A thermal recording sheet having a thermal recording layercontaining at least one urea compound (e.g. A1-A12, A31-A62) where inthe joint group is a straight-chain alkylene group having 1 to 12 carbonatoms. ##STR4## (wherein X is alkyl group having 1 to 12 carbon atoms,halogenated alkyl group having 1 to 6 carbon atoms, aralkyl group having7 to 14 carbon atoms, alkoxy group having 1 to 12 carbon atoms, aryloxygroup having 6 to 12 carbon atoms, alkoxycarbonyl group having 1 to 12carbon atoms, acyl group having 1 to 12 carbon atoms, dialkylamino grouphaving 1 to 12 carbon atoms, arylalkylamino group having 7 to 12 carbonatoms, arylamino group having 6 to 12 carbon atoms, acylamino grouphaving 1 to 12 carbon atoms, nitro group, cyano group, halogen group, orhydrogen group n is an integer from 1 to 3, and m is an integer from 1to 12).

(2) A thermal recording sheet having a thermal recording layercontaining at least one urea compound (e.g. A13-A18) wherein the jointgroup is an alkylene group having 1 to 15 carbon atoms and having abranched chain.

(3) A thermal recording sheet having a thermal recording layercontaining at least one urea compound (e.g. A19-A23, A27-A30) whereinthe joint group is a plurality of alkylenes group having 1 to 12 carbonatoms linked with nitrogen or oxygen.

The plurality of alkylenes linked with nitrogen or oxygen include thefollowing structures:

Linked with nitrogen:

    --C(CH.sub.2).sub.L1 --NH--(CH.sub.2).sub.M1 --

wherein L1 is an integer from 1 to 11, M1 is an integer from 1 to 11,and L1+M1=2 to 12.

    --(CH.sub.2).sub.L2 --NH--(CH.sub.2).sub.M2 --NH--(CH.sub.2).sub.N2 --

wherein L2 is an integer from 1 to 10, M2 is an integer from 1 to 10, N2is an integer from 1 to 10, and L2+M2+N2=3 to 12.

    --CH.sub.2).sub.L3 --NH--CH.sub.2).sub.M3 --NH--CH.sub.2).sub.N3 --NH--CH.sub.2).sub.Q3 --

wherein L3 is an integer from 1 to 9, M3 is an integer from 1 to 9, N3is an integer from 1 to 9, Q3 is an integer from 1 to 9, andL3+M3+N3+Q3=4 to 12.

    --CH.sub.2).sub.L4 --(NH--CH.sub.2).sub.M4).sub.3 --NH--CH.sub.2)N.sub.4 --

wherein L4 is an integer from 1 to 8, M4 is an integer from 1 to 2, N4is an integer from 1 to 8, and L4+3×M4+N4=5 to 12

    --(CH.sub.2).sub.L5 --(NH--(CH.sub.2).sub.m5).sub.4 --NH--(CH.sub.2).sub.n5 --

( wherein L5 is an integer from 1 to 7, M5 is an integer from 1 to 2, N5is an integer from 1 to 7 , and L5+4×M5+N5=6 to 12.)

Linked with oxygen

    --(CH.sub.2).sub.L6 --O--(CH.sub.2).sub.M6 --

wherein L6 is an integer from 1 to 11, M6 is an integer from 1 to 11,and L6+M6=2 to 12.

    --(CH.sub.2).sub.L7 --O--(CH.sub.2).sub.M7 --O--(CH.sub.2).sub.N7 --

(wherein L7 is an integer from 1 to 10, M7 is an integer from 1 to 10,N7 is an integer from 1 to 10, and L7+M7+N7=3 to 12.)

    --(CH.sub.2).sub.L8 --O--(CH.sub.2).sub.m8 --O--(CH.sub.2).sub.N8 --O--(CH.sub.2).sub.Q8 --

wherein L8 is an integer from 1 to 9, M8 is an integer from 1 to 9, N8is an integer from 1 to 9, Q8 is an integer from 1 to 9, andL8+M8"N8+Q8=4 to 12.

When linked with nitrogen, hydrogen linked to the nitrogen atom of thejoint group may be substituted with alkyl group of 1 to 12 carbon atomsor aryl group of 6 to 18 carbon atoms or unsubstituted.

(4) A thermal recording sheet having a thermal recording layercontaining at least one urea compound (e.g. A103, A104, A114, A115)wherein the joint group is a substituted or unsubstituted cycloalkylgroup having 1 to 20 carbon atoms.

This is further divided into one in which the two phenylurea structuresare linked directly to a single cycloalkyl ring, one in which the twophenylurea structures are linked to two cycloalkyls, and one in whichthe two phenylurea structures are linked to alkylene group bonded to acycloalkyl ring.

(5) A thermal recording sheet having a thermal recording layercontaining at least one urea compound (e.g. A87-A102, A107, A108)wherein the joint group is a group having one substituted orunsubstituted aromatic ring and having 1 to 20 carbon atoms.

This is further divided into one in which the two phenylurea structuresare linked directly to one aromatic ring, and one in which the twophenylurea structures are linked to alkylene group bonded to an aromaticring. The aromatic ring can be benzene ring, pyridine ring, naphthalenering, and the like, and benzene ring is considered to be preferable inview of availability of the raw material.

(6) A thermal recording sheet having a thermal recording layercontaining at least one urea compound (e.g. A63-A86, A197-A199) whereinthe joint group is a group having at least two substituted orunsubstituted aromatic rings and having 1 to 30 carbon atoms.

This is further divided into those having two aromatic rings, threearomatic rings, and four aromatic rings. Also in this case, benzene ringis preferable in view of availability of the raw material.

However, of the urea compounds of this category, the urea compound ofFormula (6), that is, the joint group is diphenylene group, isconsidered to be carefully used for general applications in view ofsafety of the raw material for synthesis. ##STR5## wherein X is alkylgroup having 1 to 12 carbon atoms, halogenated alkyl group having 1 to 6carbon atoms, aralkyl group having 7 to 14 carbon atoms, alkoxy grouphaving 1 to 12 carbon atoms, aryloxy group having 6 to 12 carbon atoms,alkoxycarbonyl group having 1 to 12 carbon atoms, acyl group having 1 to12 carbon atoms, dialkylamino group having 1 to 12 carbon atoms,arylalkylamino group having 7 to 12 carbon atoms, arylamino group having6 to 12 carbon atoms, acylamino group having 1 to 12 carbon atoms, nitrogroup, cyano group, halogen group, or hydrogen group n is an integerfrom 1 to 3).

(7) A thermal recording sheet having a thermal recording layercontaining at least one urea compound (e.g. A109-A111) wherein the jointgroup contains a substituted or unsubstituted piperazine ring and having4 to 12 carbon atoms.

In terms of the recording density, a case where the joint group is astraight-chain alkylene group having 1 to 12 carbon atoms, a case wherethe joint group is an alkylene group having 1 to 15 carbon atoms andhaving a branched chain, and a case where the joint group is a pluralityof alkylene group having 1 to 12 carbon atoms linked with nitrogen oroxygen atom are preferable. And in view of the total balance of therecording density and heat resistance, a case where the joint group is astraight-chain alkylene group having 1 to 12 carbon atoms, and a casewhere the joint group is an alkylene group having 1 to 15 carbon atomsare more preferable.

Further, the sustituent (X) of phenyl urea structure of Formula (1) isalso to be selected according to the melting point, decompositiontemperature, the easy handling in the synthesis, and solubility insolvents of the urea compound, and the property of the thermal recordingmaterials using the urea compound (recording density, heat resistance,and so on.), as for joint group, and is not specifically limited. Forinstance, in view of the recording density, a case where the substitutedis hydrogen atom (that is, unsubstituted) and a case where thesubstituent is an electorophilic group (e.g. halogenated alkyl group,nitro group, cyano group, halogen atom) are preferable. And in terms ofthe cost and safety of the urea compound, a case where the substitutedis hydrogen atom is considered to be more preferable.

On the other hand, the thermal recording sheet using the dimerized ureacompound of Formula (2) or (3) or that using the trimerized ureacompound of Formula (4) of the present invention is superior in groundcolor stability to heat and solvents as basic properties. That is, evenwhen the thermal recording sheet is placed in a high-temperatureenvironment of above 120° C, the ground color of the recording surfaceis substantially unchanged. For example, the exemplified compound A8does not develop a color even at 150° C. which is the highesttemperature of static color development. However, when the compound isapplied with high energy of the thermal head of normally at 200° to 300°C., it develops a dense color. For the heat resistance of ground color,in the common sense on thermal recording sheet in the past, it cannot beconsidered to be possible that no color is developed when the surface ofthe thermal recording sheet is applied with a heat block at above 120°C., but can be recorded with a practically usable density by a thermalhead, and such a material has been unknown.

Since the thermal recording sheet of the present invention has theabove-described high heat resistance, it can be heat laminated with aplastic film on the recording surface after recording, used as anelectrophotographic transfer sheet to be thermally fixed by coating atoner on the surface of the recording layer, and thermally fixed bycoating a toner on the recording surface of the recorded sheet.

Further, the thermal recording sheet of the present invention is veryeasy in controlling the production process. In the production ofconventional thermal recording sheet, the drying process after coatingthe recording layer requires very strict temperature control to preventthe coated surface from color development, limiting high-speed coating.However, since the thermal recording sheet of the present invention doesnot develop the ground color even when applied with hot air at 120° C.it can be dried at a high temperature, and the control range of thedrying temperature can be substantially expanded, a remarkableimprovement in productivity can be expected.

The thermal recording sheet of the present invention, possibly due tovery low solubility in organic solvents of the dimerized or trimerizedurea compound used, causes almost no change in ground color even whencontacted with solvents other than alcohol. Therefore, it can berecorded by oil ink on the recording surface.

The inventors have conducted studies on the thermal decompositiontemperature and solubility to solvents of a number of dimerized ortrimerized urea compounds, and have found that the thermal decompositiontemperature and solubility to solvents can be controlled by selectingthe joint group. By further studies, they have found a recording sheetin which the recorded image can be erased by applying an amount of heatby a heat roll, a thermal head, a drying oven or a hot stamp, byirradiation with light by a laser or a halogen lamp, or by contactingwith alcoholic solvents such as methanol or ethanol, and the erasedsurface can be recorded again by applying a thermal head or laser, thatis, a reversible recording sheet. This is achieved by using at least onedimerized urea compound of Formula (2) or (3) or a trimerized ureacompound of Formula (4) as a color developer.

A reversible recording sheet which is good in erasability by heat rolluses a color developer of a urea compound of Formula (5) (e.g. A1-A12)wherein the joint group is straight-chain alkylene group having 1 to 12carbon atoms.

The thermal recording sheet using these urea compounds as a colordeveloper, after being color developed by a thermal head or the like,can be erased by contacting with a heat roll at 100° to 200° C., and theerased surface can be recorded again by a thermal head or the like. Forexample, good results are obtained with a recording sheet which ues theexemplified compounds such as A3, A4, A5, A6, A8, A10, A14, and A49 as acolor developer.

The dimerized urea compound of Formula (3) and the trimerized ureacompound of Formula (4), from another point of view, are considered ascompounds in which the phenylurea structures are appropriately separatedby the joint group. From this point of view, a urea compound which issuperior in erasing ground color under a heating condition differentfrom recording by a thermal head is preferably a dimerized urea compoundin which the phenylurea structures are separated by 3 to 8 carbon atoms.

The dimerized or trimerized urea compound used in the present inventionalone in itself has a high color developing ability as a color developerin the thermal recording sheet, and the surface of the substrate coatedwith the compound along with the electron-donating dye precursor doesnot substantially develop a color even at 120° C., showing basicproperties of superior solvent resistance of ground color. In addition,depending on the selected joint group, some types of the urea compoundof the present invention have a reversible recording function (colordeveloping/erasing). Such characteristics are new characteristics whichhave been unknown in the past, and these urea compounds are superiormaterials that have both color developing and erasing functions thoughit is a single color developer.

In a general method for producing the thermal recording sheet of thepresent invention, (a) the dye precursor and (b) the dimerized ortrimerized urea compound as a color developer are individually dispersedalong with a binder having a dispersing function and, as necessary,mixed with additives such as a filler, a slip agent, and the like toobtain a coating color, which is coated on a substrate by a conventionalmethod known in the art, and then dried. The dimerized or trimerizedurea compounds of the present invention can be used alone or incombination.

The dye precursor used in the thermal recording sheet of the presentinvention can be those which are known to the public in the area ofthermal recording, and is not specifically limited, but triphenylmethanetype leuco dyes, fluorane type leuco dyes, fluorene type leuco dyes, andthe like are preferable. Typical dye precursors are shown below:

3,3-Bis(4'-dimethylaminophenyl)-6-dimethylaminophthalide [alias CrystalViolet Lactone (CVL)]

3,3-Bis(4'-dimethylaminophenyl)-6-pyrrolidylphthalide

3,3-Bis(4'-dimethylaminophenyl)phthalide [alias Malachite Green Lactone(MGL)]

Tris(4-(dimethylamino)phenyl)methane (alias Leuco Crystal Violet (LCV))

3-Dimethylamino-6-methyl-7-(m-trifluoromethylanilino) fluorane

3-Diethylamino-6-methyl-fluorane

3-Diethylamino-7-methyl-fluorane

3-Diethylamino-7-chloro fluorane

3-Diethylamino-6-methyl-7-chlorofluorane

3-Diethylamino-6-methyl-7-anilinofluorane

3-Diethylamino-6-methyl-7-p-methylanilinofluorane

3-Diethylamino-6-methyl-7-(o,p-dimethylanilin)fluorane

3-Diethylamino-6-methyl-7-(m-trifluoromethylanilino)fluorane

3-Diethylamino-6-methyl-7-(o-chloroanilino)fluorane

3-Diethylamino-6-methyl-7-(p-chloroanilino)fluorane

3-Diethylamino-6-methyl-7-(o-fluoroanilino)fluorane

3-Diethylamino-6-methyl-7-(p-n-butylanilino)fluorane

3-Diethylamino-6-methyl-7-n-octylaminofluorane

3-Diethylamino-6-chloro -7-anilinofluorane

3-Diethylamino-6-ethoxyethyl-7-anilinofluorane

3-Diethylamino-benzo[a]fluorane

3-Diethylamino-benzo[c]fluorane

3-Diethylamino-6-methyl-7-benzylaminofluorane

3-Diethylamino-6-methyl-7-dibenzylaminofluorane

3-Diethylamino-7-di(p-methylbenzyl)aminofluorane

3-Diethylamino-6-methyl-7-diphenylmethylaminofluorane

3-Diethylamino-7-dinaphthylmethylaminofluorane

10-Diethylamino-4-dimethylaminobenzo[a ]fluorane

3-Dibutylamino-6-methylfluorane

3-Dibutylamino-6-methyl-7-chlorofluorane

3-Dibutyiamino-6-methyl-7-anilinofluorane

3-Dibutylamino-6-methyl-7-p-methylanilinofluorane

3-Dibutylamino-6-methyl-7-(o,p-dimethylanilino)fluorane

3-Dibutylamino-6-methyl-7-(m-trifluoromethylanilino)fluorane

3-Dibutylamino-6-methyl-7-(o-chloroanilino)fluorane

3-Dibutylamino-6-methyl-7-(p-chloroanilino)fluorane

3-Dibutylamino-6-methyl-7-(o-fluoroanilino)fluorane

3-Dibutylamino-6-methyl-7-)p-n-butylanilino)fluorane

3-Dibutylamino-6-methyl-7-n-octylaminofluorane

3-Dibutylamino-6-chloro-7-anilinofluorane

3-Dibutylamino-6-ethoxyethyl-7-anilinofluorane

3-Di-n-pentylamino-6-methyl-7-anilinofluorane

3-Di-n-pentylamino-6-methyl-7-(o,p-dimethylanilino)fluorane

3-Di-n-pentylamino-6-methyl-7-(m-trifluoromethylanilino)fluorane

3-Di-n-pentylamino-6-methyl-7-(o-chloroanilino)fluorane

3-Di-n-pentylamino-6-methyl-7-(p-chloroanilino)fluorane

3-Di-n-pentylamino-6-methyl-7-(o-fluoroanilino)fluorane

3-Pyrrolidino-6-methyl-7-anilinofluorane

3-Piperidino-6-methyl-7-anilinofluorane

3-(N-methyl-N-n-propylamino)-6-methyl-7-anilinofluorane

3-(N-ethyl-N-n-propylamino)-6-methyl-7-anilinofluorane

3-(N-ethyl-N-isopropylamino)-6-methyl-7-anilinofluorane

3-(N-ethyl-N-n-butylamino)-6-methyl-7-anilinofluorane

3-(N-ethyl-N-isobutylamino)-6-methyl-7-anilinofluorane

3-(N-ethyl-N-n-hexylamino)-6-methyl-7-p-methylanilinofluorane

3-(N-ethyl-N-n-hexylamino)-6-methyl-7-(o,p-dimethylanilino)fluorane

3-(N-ethyl-N-n-hexylamino)-6-methyl-7-(m-trifluoromethylanilino)fluorane

N3-(N-ethyl-N-n-hexylamino)-6-methyl-7-(o-chloroanilino)fluorane

3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilino)fluorane

3-(N-ethyl-N-isoamylamino)-6-chloro-7-anilino)fluorane

3-(N-ethyl-N-3-methylbutylamino)-6-methyl-7-anilino)fluorane

3-(N-ethyl-N-p-toluidino)-6-methyl-7-anilinofluorane

3-(N-ethyl-N-p-toluidino)-6-methyl-7-(p-methylanilino)fluorane

3-(N-ethyl-N-p-toluidino)-6-methyl-7-(o,p-dimethylanilino)fluorane

3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluorane

3-(N-cyclohexyl-N-methylamino)-6-methyl-7-anilinofluorane

3-(N-cyclohexyl-N-methylamino)-7-anilinofluorane

3-(N-ethyl-N-3-methoxypropylamino)-6-methyl-7-anilinofluorane

3-(N-ethyl-N-3-ethoxypropylamino)-6-methyl-7-anilinofluorane

2-(4-Oxahexyl)-3-dimethylamino-6-methyl-7-anilinofluorane

2-(4-Oxahexyl)-3-diethylamino-6-methyl-7-anilinofluorane

2-(4-Oxahexyl)-3-dipropylamino-6-methyl-7-anilinofluorane

3-(4"-Aminostilbuldyl-4'-amino)-7,8-benzofuran

3,6,6'-Tris(dimethylamino)spiro[fluorene-9,3'-phthalide]

3,6,6'-Tris(diethylamino)spiro[fluorene-9,3'-phthalide]

3-(4-Diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide

3-(4-Diethylamino-2-ethoxyphenyl)-3-(1-octyl-2-methylindol-3-yl)-4-azaphthalide

3-(4-Diethylamino-2-n-hexylphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide

3-(4-Cyclohexylmethylamino-2-methoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide

3-(4-Cyclohexylethylamino-2-methoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide

3,3-Bis(1-ethyl-2-methylindol-3-yl)phthalide

3,3-Bis(1-methyl-2-octylindol-3-yl)phthalide

3-(1-Ethyl-2-methylindol-3-yl)-3-(1-n-butyl-2-methylindol-3-yl)phthalide

3,7-Bis(dimethylamino)-10-benzoylphenothiazine

3,7-Bis(dimethylamino)-N-[p-N-bis(4,4'-dimethylaminophenyl)methylamino]benzoylphenothiazine

3,7-Bis(dimethylamino)-N-(p-N-bis(4,4'-diethylaminophenyl)methyl)benzoylphenothiazine

3,6-Bis(diethylamino)fluorane-γ-(2'-nitro)anilinolactam

3,6-Bis(diethylamino)fluorane-γ-(3'-nitro)anilinolactam

3,6-Bis(diethylamino)fluorane-γ-(4'-nitro)anilinolactam

3,6-Bis(diethylamino)fluorane-γ-anilinolactam.

These dye precursors may be used alone or as mixtures of two or moretypes. Since fluorane type dye precursors are particularly high instability of color development and ground color as basic functions ofthe thermal recording sheet, they can be preferably used in the presentinvention. Naturally, when thermal stability is important, a dyeprecursor which is high in melting point and decomposition temperatureis preferable. When the reversible recording function is important, itis preferable to use3-dimethylamino-7-(m-trifluoromethylanilino)fluorane and the like, whichis weak in image stability, can withstand repeated use, and is high indecomposition temperature.

The binder usable in the present invention includes completelyhydrolyzed polyvinylacohol having a polymerization degree of 200 to1900, partially-hydrolyzed polyvinylalcohol, carboxy-modifiedpolyvinylalcohol, amide-modified polyvinylalcohol, sulfonicacid-modified polyvinylalcohol, butyral-modified polyvinylalcohol, othermodified polyvinylalcohols, styrene-maleic anhydride copolymer,styrene-butadiene copolymer, cellulose derivatives such ashydroxyethylcellulose, methylcellulose, carboxymethylcellulose,ethylcellulose and acetylcellulose, polyvinylchloride, polyvinylacetate,polyacrylamide, polyacrylic esters, polyvinylbutyral, polystyrene andcopolymers thereof, polyamide resins, silicone resins, petroleum resins,terpene resins, ketone resins, and coumarone resins. Of these binders,the polyvinylalcohol type binder is preferable in terms of thedispersibility, binding properties, and thermal stability of groundcolor. For reversible recording, a binder which can with stand repeateduse and is small in degradation is preferable. These binders are used bydissolving in solvents such as water, alcohol, ketone, ester, andhydrocarbon, emulsifying in water or other solvents, or dispersing to apaste form, and can be used in combination according to the qualityrequirements.

In the present invention, when a thermal recording sheet which isparticularly high in thermal stability of ground color is produced, itis better in principle not to use a sensitizer. When a sensitizer isused, it tends to melt at a drying temperature, and causes the dyeprecursor to react with the color developer to develop the ground color.However, on the other hand, since the sensitizer promotes the erasingfunction, when importance is attached to the erasing function ratherthan the thermal stability of ground color, that is, the reversiblerecording function, a sensitizer may be used. Sensitizers used for thispurpose include 2-di(3-methylphenoxy)ethane, p-benzylbiphenyl,β-benzyloxynaphthalene, phenyl 1-hydroxy-2-naphthoate,dibenzylterephthalate, benzyl p-benzyloxybenzoate, diphenylcarbonate,ditolylcarbonate, 4-biphenyl-p-tolylether, m-terphenyl,1,2-diphenoxyethane, 1,2-bis(m-tolyloxy)ethane,di(p-methylbenzyl)oxalate, and di(p-chlorobenzyl)oxalate.

The filler used in the present invention includes inorganic fillers suchas silica, calcium carbonate, kaolin, calcined kaolin, diatomaceousearth, talc, zinc oxide, titanium oxide, zinc hydroxide, and aluminumhydroxide; polystyrene-based organic fillers, styrene/butadiene-basedorganic fillers, styrene/acrylic-based organic fillers, and holloworganic fillers.

In addition to the above, a release agent such as fatty acid metalsalts, a slip agent such as waxes, benzophenoe or benzotriazole typeultraviolet absorbents, a water-resistant agent such as glyoxal, adispersant, a defoamer, and the like can be used.

Types and ratios of (a) the dye precursor, (2) the dimerized ortrimerized urea compound, and other ingredients are determined by therequired properties and recording adaptability, and are not specificallylimited but, normally, based on one part of the dye precursor, 1 to 8parts of the dimerized or trimerized urea compound, and 1 to 20 parts ofthe filler are used, and the binder is used in an amount of 10 to 25% byweight to the total solid. For repeated use such as the reversiblerecording sheet, it is preferable to use a composition as simple aspossible. These materials are finely crushed by a crusher such as a ballmill, an attriter, or a sand grinder, or an appropriate emulsifyingapparatus to a particle diameter of several microns or less, a binderand, as necessary, other additives are added to obtain a coating color.The coating color comprising the above composition is coated on adesired substrate such as paper, synthetic paper, non-woven fabrics,plastic films, Diastic sheets, or composite sheets thereof to obtain theobjective thermal recording sheet.

Further, an overcoating layer comprising a polymer can be provided ontop of the recording layer to enhance the preservability, or anundercoating layer of a polymer containing a filler can be providedunder the recording layer to enhance the color developing sensitivity.

The thermal recording sheet of the present invention, utilizing its highground color stability, may be heat laminated with a plastic film toform a transparent and strong protective coating. For example, evenafter thermal recording, a heat-resistant card can be easily preparedusing a commercial simple laminator.

Of the thermal recording sheets of the present invention, one which hasan erasing function is useful as a reversible recording sheet (rewriteor rewritable recording sheet), or as a simple displaying sheet.However, for the latter application, it is necessary to achieverecording and erasing almost simultaneously.

Erasing of the thermal recording sheet of the present invention isachieved by two methods. One method uses a heat roll, a thermal head, ahot stamp, a carbon dioxide laser, a semiconductor laser, sunlight, ahalogen lamp, or the like to erase the recorded image by heat. Forexample, for a heat roll, the erasing temperature is preferably 100° to200° C. with a feed speed of 8 to 45 mm/sec. The other method uses analcoholic solvent to erase the recorded image.

The thermal recording sheet of the present invention may contain anoptical absorbent which absorbs light to convert it to heat in thethermal recording layer, for use as an optical recording sheet. Theoptical absorbent can be a substance which absorbs wavelengths ofvarious light sources, and is not specifically limited.

For example, for a recording light source having continuous wavelength,such as a stroboflash, the optical absorbent can be a heat reactionproduct of thiourea derivative/copper compound described in JapaneseOPIs 2-206583 and 5-30954, graphite, copper sulfide, molybdenumtrisulfide, black titanium, and the like described in Japanese OPI3-86580, or carbon black.

On the other hand, when a semiconductor laser is used as a recordinglight source, the optical absorbent can be polymethine type dyes(cyanine dyes); azolenium type dyes, pylylium type dyes, thiopylyliumtype dyes, squalylium type dyes, chroconium type dyes, dithiolcomplexes, mercaptophenol-metal complex type dyes,mercaptonaphthol-metal complex type dyes, phthalocyanine type dyes,naphthalocyanine type dyes, triarylmethane type dyes, immonium typedyes, diimmonium type dyes, naphthoquinone type dyes, anthraquinone typedyes, and metal complex type dyes. Further, the optical absorbentslisted for a light source having continuous wavelength can also be usedas well.

Specifically, a near infrared absorption dyes described in "Colorchemical dictionary" (The society of Synthetic Organic Chemistry,CMC-publishing company, 1988, P196-200). "Kagaku to Kogyo" (Vol. 5,1986, p376-389). Japanese OPIs 61-69991, and 61-246391, and U.S. Pat.Nos. 3,251,881, 35,570,122, 3,575,871, and 3,637,769;1,1,5,5-tetrakis(p-dimethylaminophenyl)-3-methoxy-1,4-pentadien (or itscation form), 1,1,5,5-tetrakis(p-diethylaminophenyl)-3-methoxy-1,4-pentadien (or its cation form); toluenedithiol nickel complex,4-tert-butyl-1,2-benzenedithiol nickel complex,bisdithiobenzilnickelcomplex, bis(4-ethyldithiobenzil)nickel complex,and so on. These optical absorbents can be used alone or as mixture oftwo or more types.

These optical absorbents may be used by: (a) a method in which theoptical absorbent is simply mixed in the materials necessary for thethermal recording sheet, (b) a method in which the optical absorbent ispreviously melted and mixed, and dissolved or dispersed to be used, asdescribed in Japanese OPI 2-217287, or (c) a method in which the opticalabsorbent is previously dissolved or dispersed by a solvent in thematerials necessary for the thermal recording sheet, the solvent isremoved from the mixture, and then used. The optical absorbent may alsobe co-dispersed with the color developer with the dre precuesor, withthe color developer and the sensitizer or with the dye precursor and thesensitizer.

The thermal recording sheet of the present invention, even when combinedwith the optical absorbent to form an optical recording sheet, issubstantially unchanged in the basic properties (e.g. ground colorstability such as heat resistance and solvent resistance). Even theoptical recording sheet can be heat laminated or toner recorded. This isalso true for the additional functions (reversible recording). However,when a sensitizer is used, the heat resistance tends to be impaired. Thethermal recording sheet laminated with the plastic film, which containsthe optical absorbant, can be recorded by light and as laser-light.

The novel dimerized or trimerized urea compound according to the presentinvention is a color developer which is superior in color developmentand ground color stability to heat and solvents as basic functions, andsome types have a reversible recording function. The reason for thesuperior ground color stability and the reversible recording functionhas yet to clarified. However, this is considered as follows:

Depending on the condition, the dimerized or trimerized urea compound ofthe present invention changes in structure as shown below. Since thischange is a phenomenon similar to the keto/enol tautomerism, and isreferred here to as keto-form and enol-form for convenience. ##STR6##

It is considered that the dimerized or trimerized urea compound musthave the enol-form to function as a color developer. Sinceenol-formation requires a high temperature, and the thermal headinstantaneously provides a high temperature of 200 to 3000 C., the ureacompound contacting with the thermal head undergoes enol-formation tohave a color developing function, opening the lactone ring of the dyeprecursor to develop a color. Therefore, the urea compound does notchange until the enol-formation temperature is reached, does not reactwith the dye precursor, and the ground color remains unchanged. Thiswould be the reason for the high heat resistance. Further, since thedimerized or trimerized urea compound increases the number of activehydrogens compared to a monourea compound, a good color developingfunction is obtained.

On the other hand, if the thus produced enol-form converts to theketo-form for some reason, erasing would occur. Therefore, when thecompound is provided with an appropriate temperature and head, orcontacts with an alcoholic solvent, keto-formation occurs, resulting inerasing. Since enol-formation and keto-formation occur under quitedifferent conditions, the enol-form and the keto-form can be repeatedunder the individual conditions, thereby enabling reversible recording.

The reason why the ground color is not changed by writing with an oilink is considered as due to the fact that the dimerized or trimerizedurea compound of the present invention is very low in solubility in thesolvent used in the oil ink, and the dye precursor and the colordeveloper are not substantially mixed with each other even whencontacting the solvent.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS EXAMPLES

Synthesis of dimerized and trimerized urea compounds

Dimerized and trimerized urea compounds were synthesized by thefollowing synthetic methods. General synthetic methods are shown below.

Synthesis Example 1

A monoisocyanate compound (2.2 eq.) was dissolved in ethyl acetate (oracetone). Into the solution, a solution of a diamine compound (1.0 eq,)in ethyl acetate (or acetone) was dropped. After stirring for apredetermined time, generated precipitate was filtered, and washed withethyl acetate, n-hexane, hot water, and methanol in this order until 1spot was obtained on TLC (thin layer chromatograph) to obtain a ureacompound.

Synthesis Example 2

A diisocyanate compound (1.0 eq.) was dissolved in ethyl acetate (oracetone). Into the solution, a solution of a monoamine compound (2.2eq.) in ethyl acetate (or acetone) was dropped. After stirring for apredetermined time, generated precipitate was filtered, and washed withethyl acetate, n-hexane, hot water, and methanol in this order until 1spot was obtained on TLC to obtain a urea compound.

Synthesis Example 3

A triisocyanate compound (1.0 eq.) was dissolved in ethyl acetate (oracetone). Into the solution, a solution of a monoamine compound (3.3eq.) in ethyl acetate (or acetone) was dropped. After stirring for apredetermined time, generated precipitate was filtered, and washed withethyl acetate, n-hexane, hot water, and methanol in this order until 1spot was obtained on TLC to obtain a urea compound.

Production of thermal recording sheet

In the following description, unless otherwise noted, part and %indicate part by weight and % by weight, respectively.

EXAMPLES 1-48

As shown below, thermal recording sheets were produced using3-N,N-diethylamino-6-methyl-7-anilinofluorane (ODB) as a dye precursorand the dimerized urea compound (or trimerized urea compound) of thepresent invention (Table 1, Table 2, Table 3) as a developer.

Specifically, a color developer dispersion (Solution A) and a dyeprecursor dispersion (Solution B) of the following compositions weremilled by a sand grinder to an average particle diameter of 1 micron.

    ______________________________________                                        Solution A: color developer dispersion                                        Inventive dimerized urea compound                                                                    6.0      parts                                         (or trimerized urea compound)                                                 10% Aqueous polyvinylalcohol solution                                                                18.8                                                   Water                  11.2                                                   Solution B: dye precursor dispersion                                          3-N,N-diethylamino-6-methyl-7-                                                                       2.0      parts                                         anilinofluorane (ODB)                                                         10% Aqueous polyvinylalcohol solution                                                                4.6                                                    Water                  2.6                                                    ______________________________________                                    

Then, the Solution A (color developer dispersion), the Solution B (dMeprecursor dispersion), and a kaolin clam dispersion were mixed in thefollowing ratio to obtain a coating color.

    ______________________________________                                        Solution A (color developer dispersion)                                                              36.0     parts                                         Solution B (dye precursor dispersion)                                                                9.2                                                    Kaolin clay (50% dispersion)                                                                         12.0                                                   ______________________________________                                    

The coating color was coated on one side of a 50 g/m² base paper to acoating coverage of 6.0 g/m², dried, and supercalendered to a flatnessof 500 to 60 seconds to obtain a thermal recording sheet.

EXAMPLES 49-56

Thermal recording sheets were produced using the dye precursors otherthan ODB and using the same procedure as in Examples 1-48 (Table 5).

    ______________________________________                                        Dye precursor                                                                 ______________________________________                                        ODB-2:    3-N,N-dibutylamino-6-methyl-7-anilinofluorane                       CVL:      3,3-bis(p-dimethylaminophenyl)-6-methyl-7-                                    nilinophthalide                                                     NEW-Blue: 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-                               methylindol-3-yl)-4-azaphthalide                                    I-red:    3,3-bis(1-ethyl-2-methylindol-3-yl)phthalide                        ______________________________________                                    

First, a dye precursor dispersion (Solution C) of the followingcomposition was milled by a sand grinder to an average particle diameterof 1 micron.

    ______________________________________                                        Solution C: dye precursor dispersion                                          ______________________________________                                        Above dye precursor    2.0      parts                                         10% Aqueous polyvinylalcohol solution                                                                4.6                                                    Water                  2.6                                                    ______________________________________                                    

Then, the color developer dispersion used in Example 4 (or Example 6),Solution C, and a kaolin clay dispersion were mixed in the followingratio to obtain a coating color.

    ______________________________________                                        Color developer dispersion of Example 6 using the                                                        36.0   part                                        compound A8 (or color developer dispersion of                                 Example 4 using the compound A6)                                              Solution C: dye precursor dispersion                                                                     9.2                                                Kaolin clay (50% dispersion)                                                                             12.0                                               ______________________________________                                    

The coating color was coated on one side of a 50 g/m² base paper in anamount of 6.0 g/m² and dried, and the sheet was supercalendered to aflatness of 500 to 600 seconds to obtain a thermal recording sheet.

Comparative Examples 1-7

Thermal recording sheets for comparative tests were prepared usin g theknown compounds shown below as color developers and using the sameprocedure as in Examples 1-48.

                  TABLE 4                                                         ______________________________________                                        Known color developer compounds                                               ______________________________________                                        Bisphenol A (B1)                                                              Bisphenol S (B2)                                                              4-Hydroxy-4'-iso-propoxydiphenylsulfone (B3)                                  4-Hydroxy-4'-n-propoxydiphenylsulfone (B4)                                    Phenylurea (B5) described in Japanese OPI 58-211496                           Dimerized urea (B6) described in Japanese OPI 5-147357                        Amidephenol derivative (B7)                                                    ##STR7##                   (B1)                                               ##STR8##                   (B2)                                               ##STR9##                   (B3)                                               ##STR10##                  (B4)                                               ##STR11##                  (B5)                                               ##STR12##                  (B6)                                               ##STR13##                  (B7)                                              ______________________________________                                    

Specifically, dispersions of the above individual known color developercompounds of the following composition were milled by a sand grinder toan average particle diameter of 1 micron.

    ______________________________________                                        Solution D: color developer dispersion                                        ______________________________________                                        Known color developer compound (B1-B7)                                                                 6.0     parts                                        10% Aqueous polyvinylalcohol solution                                                                  18.8                                                 Water                    11.2                                                 ______________________________________                                    

Then, Solution D (color developer dispersion), the dye precursor (ODB)dispersion (Solution B) used in Examples 1-48, and a kaolin claydispersion were mixed in the following ratio to obtain a coating color.

    ______________________________________                                        Solution D: color developer dispersion                                                               36.0     parts                                         Solution B: dye precursor dispersion                                                                 9.2                                                    (ODB dispersion)                                                              Kaolin clay (50% dispersion)                                                                         12.0                                                   ______________________________________                                    

The coating color was coated on one side of a 50 g/m² base paper in anamount of 6.0 g/m² and dried, and the sheet was supercalendered to aflatness of 500 to 600 seconds to obtain a thermal recording sheet.

Comparative Examples 8-9

Thermal recording sheets for comparative tests were prepared usinghisphenol A as a color developer, and ODB-2 and NEW-BLUE and using thesame procedure as in Examples 49-56. (Table 5)

Evaluation of Thermal Recording Sheets

The resulting thermal recording sheets were tested for basic propertiesby a recordability test using a thermal printer, a ground color thermalstability test, and an oil ink adaptability test.

Recordability test (dynamic color developing density)

To test the recording adaptability, the thermal recording sheet wasrecorded using a word processor printer (RUPO-90F: Trade mark of wordprocessor made by Toshiba co,. ltd.) at a maximum energy, and therecorded portion was measured by a Macbeth densitometer (RD-914, anamber filter used. Hereinafter density was measured in this condition).In this case, the greater the Macbeth value, the higher the recordingdensity and the better the recording adaptability.

Ground color thermal stability test (static color developing density)

To test the thermal stability of ground color, the recording sheet waspressed against a hot plate heated individually at 90° C., 120° C. and150° C. at a pressure of 10 g/cm² for 5 seconds, and the treated sheetwas measured by a Macbeth densitometer. In this case, the smaller theMacbeth value, the smaller the coloring of ground color and the higherthe thermal stability of ground color. Oil ink adaptability test(discoloration of ground color by oil ink):

The recording sheet was written with a felt pen of red oil ink Magic INKNo. 900/Teranishi Kagaku co,. ltd., and visually measured for a degreeof discoloration compared to a conventional red ink.

A: No discoloration

B: Little discoloration

C: Slight discoloration

D: Considerable discoloration

The evaluation results of the basic properties of Examples 1-56 usingthe dimerized or trimerized urea compounds of the present invention ascolor developers, and Comparative Examples 1-9 using the conventionalcolor developer compound are shown in Tables 1 to 5.

                  TABLE 1                                                         ______________________________________                                                  Ground            Thermal stability                                 Color     color    Recorda- of ground color                                                                          Oil ink                                      deve-   before   bility 90°                                                                         120°                                                                        150°                                                                        adapta-                          Entry loper   recording                                                                              test   C.   C.   C.   bility                           ______________________________________                                        Ex. 1 A3      0.03     1.34   0.03 0.03 0.04 A                                Ex. 2 A4      0.02     1.12   0.02 0.03 0.04 A                                Ex. 3 A5      0.03     1.28   0.03 0.03 0.05 A                                Ex. 4 A6      0.02     1.26   0.02 0.03 0.04 A                                Ex. 5 A7      0.03     1.35   0.03 0.04 0.05 A                                Ex. 6 A8      0.02     1.21   0.02 0.03 0.05 A                                Ex. 7 A10     0.04     1.15   0.04 0.04 0.04 A                                Ex. 8 A12     0.02     1.16   0.02 0.03 0.08 A                                Ex. 9 A14     0.06     1.28   0.06 0.06 0.06 A                                Ex. 10                                                                              A15     0.02     1.16   0.03 0.03 0.05 A                                Ex. 11                                                                              A16     0.03     1.07   0.03 0.04 0.06 A                                Ex. 12                                                                              A17     0.02     1.29   0.02 0.04 0.24 A                                Ex. 13                                                                              A19     0.03     1.31   0.03 0.05 0.74 A                                Ex. 14                                                                              A28     0.03     1.28   0.03 0.04 0.79 A                                Ex. 15                                                                              A31     0.03     1.15   0.03 0.04 0.05 A                                Ex. 16                                                                              A32     0.03     1.24   0.03 0.04 0.05 A                                ______________________________________                                         Note: ODB used as a dye                                                  

                  TABLE 2                                                         ______________________________________                                                  Ground            Thermal stability                                 Color     color    Recorda- of ground color                                                                          Oil ink                                      deve-   before   bility 90°                                                                         120°                                                                        150°                                                                        adapta-                          Entry loper   recording                                                                              test   C.   C.   C.   bility                           ______________________________________                                        Ex. 17                                                                              A33     0.04     0.92   0.04 0.05 0.06 A                                Ex. 18                                                                              A34     0.03     0.91   0.03 0.04 0.05 A                                Ex. 19                                                                              A49     0.04     1.22   0.04 0.04 0.05 A                                Ex. 20                                                                              A50     0.03     1.16   0.03 0.03 0.05 A                                Ex. 21                                                                              A51     0.03     0.98   0.03 0.04 0.05 A                                Ex. 22                                                                              A52     0.04     1.11   0.04 0.04 0.05 A                                Ex. 23                                                                              A53     0.03     1.10   0.03 0.03 0.05 A                                Ex. 24                                                                              A54     0.05     1.35   0.05 0.06 0.16 A                                Ex. 25                                                                              A55     0.03     1.23   0.03 0.04 0.05 A                                Ex. 26                                                                              A56     0.04     1.24   0.04 0.04 0.06 A                                Ex. 27                                                                              A63     0.03     0.82   0.03 0.04 0.05 A                                Ex. 28                                                                              A64     0.04     0.87   0.05 0.05 0.05 A                                Ex. 29                                                                              A65     0.10     0.83   0.10 0.12 0.02 A                                Ex. 30                                                                              A66     0.04     0.86   0.04 0.05 0.06 A                                Ex. 31                                                                              A68     0.03     0.81   0.03 0.04 0.05 A                                Ex. 32                                                                              A69     0.04     0.93   0.04 0.05 0.07 A                                ______________________________________                                         Note: ODB used as a dye                                                  

                  TABLE 3                                                         ______________________________________                                                  Ground            Thermal stability                                 Color     color    Recorda- of ground color                                                                          Oil ink                                      deve-   before   bility 90°                                                                         120°                                                                        150°                                                                        adapta-                          Entry loper   recording                                                                              test   C.   C.   C.   bility                           ______________________________________                                        Ex. 33                                                                              A79     0.05     0.85   0.05 0.06 0.12 A                                Ex. 34                                                                              A82     0.12     0.80   0.12 0.13 0.14 A                                Ex. 35                                                                              A90     0.03     0.85   0.03 0.04 0.05 A                                Ex. 36                                                                              A92     0.03     0.85   0.03 0.03 0.03 A                                Ex. 37                                                                              A100    0.02     1.14   0.02 0.04 0.05 A                                Ex. 38                                                                              A101    0.03     1.09   0.03 0.04 0.05 A                                Ex. 39                                                                              A103    0.02     0.90   0.03 0.04 0.06 A                                Ex. 40                                                                              A104    0.02     0.92   0.02 0.04 0.28 A                                Ex. 41                                                                              A111    0.03     1.00   0.03 0.04 0.05 A                                Ex. 42                                                                              A112    0.04     0.81   0.04 0.04 0.05 A                                Ex. 43                                                                              A120    0.05     0.82   0.05 0.05 0.08 A                                Ex. 44                                                                              A124    0.02     0.74   0.03 0.04 0.05 A                                Ex. 45                                                                              A186    0.04     1.07   0.04 0.04 0.04 A                                Ex. 46                                                                              A200    0.02     1.16   0.02 0.03 0.05 A                                Ex. 47                                                                              A201    0.03     1.03   0.03 0.04 0.05 A                                Ex. 48                                                                              A203    0.04     1.03   0.05 0.07 0.09 A                                ______________________________________                                         Note: ODB used as a dye                                                  

                                      TABLE 4                                     __________________________________________________________________________                 Ground        Thermal stability                                         Color color before                                                                         Recordability                                                                        of ground color                                                                           Oil ink                                Entry  developer                                                                           recording                                                                            test   90° C.                                                                     120° C.                                                                    150° C.                                                                    adaptability                           __________________________________________________________________________    Comp. Ex. 1                                                                          B1    0.06   1.44   0.21                                                                              1.51                                                                              1.53                                                                              D                                      Comp. Ex. 2                                                                          B2    0.06   1.30   0.08                                                                              0.21                                                                              0.58                                                                              D                                      Comp. Ex. 3                                                                          B3    0.04   1.50   0.13                                                                              1.55                                                                              1.56                                                                              D                                      Comp. Ex. 4                                                                          B4    0.04   1.52   0.04                                                                              0.13                                                                              1.55                                                                              D                                      Comp. Ex. 5                                                                          B5    0.03   1.01   0.04                                                                              0.06                                                                              0.94                                                                              A                                      Comp. Ex. 6                                                                          B6    0.06   1.14   0.07                                                                              0.11                                                                              1.02                                                                              A                                      Comp. Ex. 7                                                                          B7    0.07   0.79   0.08                                                                              0.49                                                                              0.68                                                                              C                                      __________________________________________________________________________     Note: ODB used as a dye                                                  

                                      TABLE 4                                     __________________________________________________________________________                       Ground        Thermal stability                                         Color color before                                                                         Recordability                                                                        of ground color                                                                           Oil ink                          Entry  Dye   developer                                                                           recording                                                                            test   90° C.                                                                     120° C.                                                                    150° C.                                                                    adaptability                     __________________________________________________________________________    Ex. 49 ODB-2 A8    0.05   1.26   0.05                                                                              0.05                                                                              0.07                                                                              A                                Ex. 50 CVL   A8    0.03   0.82   0.03                                                                              0.04                                                                              0.05                                                                              A                                Ex. 51 NEW-Blue                                                                            A8    0.07   0.98   0.07                                                                              0.08                                                                              0.10                                                                              A                                Ex. 52 I-red A8    0.03   1.02   0.03                                                                              0.05                                                                              0.06                                                                              A                                Ex. 53 ODB-2 A6    0.05   1.30   0.05                                                                              0.06                                                                              0.07                                                                              A                                Ex. 54 CVL   A6    0.04   0.84   0.04                                                                              0.05                                                                              0.06                                                                              A                                Ex. 55 NEW-Blue                                                                            A6    0.08   0.90   0.08                                                                              0.09                                                                              0.09                                                                              A                                Ex. 56 I-red A6    0.03   0.80   0.03                                                                              0.05                                                                              0.06                                                                              A                                Com. Ex. 8                                                                           ODB-2 B1    0.05   1.30   0.27                                                                              1.31                                                                              1.36                                                                              D                                Com. Ex. 9                                                                           NEW-Blue                                                                            B1    0.04   1.34   0.29                                                                              1.33                                                                              1.39                                                                              D                                __________________________________________________________________________

Then, to test for additional properties, the thermal recording sheet wassubjected to a heat lamination test and a reversible recording test.

Heat lamination test (preparation of laminated recording sheet)

Using a simple lamination apparatus (MS Pouch H-140/Meiko Shokai), thethermal recording sheet was put between pouch films to prepare alaminated thermal recording sheet, and the ground color thereof wasmeasured by a Macbeth densitometer. In this case, the smaller theMacbeth value shows the better the ground color stability. In otherwords, the sheet can be laminated without coloring. The thermalrecording sheet using the dimerized or trimerized urea compound of thepresent invention was able to be laminated with a stable ground color.

Reversible recording test

As in the recordability test, the thermal recording sheet was recordedby a word processor printer, the recorded sheet was passed between 180°C. heat rolls at a speed of 30 mm/sec, and the recorded portion and theground color portion were measured by a Macbeth densitometer. In thiscase, the smaller the Macbeth value of the recorded portion shows thehigher the erasability. After that, the sheet was again recorded by theword processor printer, and the recorded portion was measured forMacbeth density.

Evaluation results of additional properties of the Examples using thedimerized or trimerized urea compound of the present invention as acolor developer and Comparative Examples 1-8 using a known colordeveloper compound are shown in Tables 8 to 9.

                                      TABLE 6                                     __________________________________________________________________________              Heat  Reversible recordability test                                 Color     lamination                                                                          Recorded                                                                            Erasing          Re-                                    Entry                                                                             developer                                                                           test  portion                                                                             (Recorded portion                                                                      Ground color)                                                                         recording                              __________________________________________________________________________    Ex. 1                                                                             A3    0.10  1.34  0.20     0.04    1.31                                   Ex. 2                                                                             A4    0.11  1.12  0.13     0.04    1.09                                   Ex. 3                                                                             A5    0.12  1.28  0.18     0.04    1.24                                   Ex. 4                                                                             A6    0.11  1.26  0.16     0.04    1.24                                   Ex. 5                                                                             A7    0.12  1.35  0.19     0.05    1.27                                   Ex. 6                                                                             A8    0.11  1.21  0.14     0.04    1.19                                   Ex. 7                                                                             A10   0.12  1.15  0.14     0.04    1.10                                   Ex. 8                                                                             A12   0.11  1.16  0.32     0.16    1.11                                   Ex. 9                                                                             A14   0.14  1.28  0.13     0.06    1.20                                   Ex. 10                                                                            A15   0.10  1.16  0.37     0.04    1.11                                   Ex. 15                                                                            A31   0.12  1.15  0.20     0.04    1.10                                   Ex. 16                                                                            A32   0.12  1.24  0.19     0.04    1.18                                   Ex. 17                                                                            A33   0.13  0.92  0.22     0.06    0.88                                   Ex. 18                                                                            A34   0.10  0.91  0.13     0.04    0.86                                   Ex. 19                                                                            A49   0.14  1.22  0.19     0.04    1.19                                   __________________________________________________________________________     Note: ODB used as a dye                                                  

                                      TABLE 7                                     __________________________________________________________________________              Heat  Reversible recordability test                                 Color     lamination                                                                          Recorded                                                                            Erasing          Re-                                    Entry                                                                             developer                                                                           test  portion                                                                             (Recorded portion                                                                      Ground color)                                                                         recording                              __________________________________________________________________________    Ex. 20                                                                            A50   0.15  1.16  0.20     0.05    1.11                                   Ex. 21                                                                            A51   0.12  0.96  0.40     0.05    0.93                                   Ex. 22                                                                            A52   0.13  1.11  0.22     0.04    1.05                                   Ex. 23                                                                            A53   0.14  1.10  0.21     0.05    1.02                                   Ex. 25                                                                            A55   0.11  1.23  0.28     0.05    1.17                                   Ex. 27                                                                            A63   0.13  0.82  0.50     0.06    0.78                                   Ex. 28                                                                            A64   0.14  0.87  0.50     0.05    0.83                                   Ex. 31                                                                            A68   0.12  0.81  0.44     0.05    0.79                                   Ex. 33                                                                            A79   0.12  0.85  0.40     0.09    0.81                                   Ex. 35                                                                            A90   0.12  0.79  0.35     0.05    0.72                                   Ex. 37                                                                             A100 0.12  1.04  0.44     0.06    0.99                                   Ex. 38                                                                             A101 0.12  1.09  0.43     0.04    1.01                                   Ex. 43                                                                             A120 0.16  0.72  0.30     0.08    0.70                                   __________________________________________________________________________     Note: ODB used as a dye                                                  

                                      TABLE 8                                     __________________________________________________________________________                       Reversible recordability test                                     Color Heat  Recorded                                                                           Erasing          Re-                                  Entry  developer                                                                           lamination                                                                          portion                                                                            (Recorded portion                                                                      Ground color)                                                                         recording                            __________________________________________________________________________    Comp. Ex. 1                                                                          B1    1.96  1.44 1.58     1.50    --                                   Comp. Ex. 2                                                                          B2    0.37  1.30 1.43     0.93    --                                   Comp. Ex. 3                                                                          B3    1.86  1.50 1.57     1.54    --                                   Comp. Ex. 4                                                                          B4    0.28  1.52 1.54     1.49    --                                   Comp. Ex. 5                                                                          B5    0.29  1.03 1.05     0.96    --                                   Comp. Ex. 6                                                                          B6    0.28  1.14 1.11     0.99    --                                   Comp. Ex. 7                                                                          B7    0.64  0.79 0.44     0.15    0.73                                 __________________________________________________________________________     Note: ODB used as a dye                                                  

                                      TABLE 9                                     __________________________________________________________________________                  Color Heat  Revers. recordability test                                                                              Re-                       Entry  Dye    developer                                                                           lamination                                                                          Recorded-Erasing                                                                       (Recorded portion                                                                      Ground color)                                                                         recording                 __________________________________________________________________________    Ex. 49 ODB-2  A8    0.09  1.26     0.10     0.05    1.25                      Ex. 50 CVL    A8    0.10  0.82     0.11     0.05    0.80                      Ex. 51 NEW-BLUE                                                                             A8    0.10  0.98     0.09     0.05    0.98                      Ex. 52 1-red  A8    0.11  1.02     0.15     0.06    0.98                      Comp. Ex. 8                                                                          ODB-2  B1    1.84  1.30     1.56     1.48    --                        Comp. Ex. 9                                                                          NEW-BLUE                                                                             B1    1.79  1.34     1.49     1.43    --                        __________________________________________________________________________

EXAMPLE 57

The thermal recording sheet of Example 1 was subjected to 100repetitions of a reversibility test by a heat roll. The Macbethdensities of the recorded portion and ground color of the 100th timewere 1.20 and 0.07, respectively.

EXAMPLE 58

The thermal recording sheet of Example 3 was subjected to 100repetitions of a reversibility test by a heat roll. The Macbethdensities of the recorded portion and ground color of the 100th timewere 1.18 and 0.07, respectively.

EXAMPLE 59

The thermal recording sheet of Example 4 was subjected to 100repetitions of a reversibility test by a heat roll. The Macbethdensities of the recorded portion and ground color of the 100th timewere 1.04 and 0.08, respectively.

EXAMPLE 60

The thermal recording sheet of Example 6 was subjected to 100repetitions of a reversibility test by a heat roll. The Macbethdensities of the recorded portion and ground color of the 100th timewere 1.09 and 0.08, respectively.

EXAMPLE 61

The thermal recording sheet of Example 49 was subjected to 100repetitions of a reversibility test by a heat roll. The Macbethdensities of the recorded portion and ground color of the 100th timewere 1.12 and 0.07, respectively.

EXAMPLE 62

The thermal recording sheet of Example 51 was subjected to 100repetitions of a reversibility test by a heat roll. The Macbethdensities of the recorded portion and ground color of the 100th timewere 0.94 and 0.09, respectively.

EXAMPLE 63

The thermal recording sheet of Example 56 was subjected to 50repetitions of a reversibility test by a heat roll. The Macbethdensities of the recorded portion and ground color of the 50th time were0.76 and 0.11, respectively.

EXAMPLE 64

The thermal recording sheet of Example 3 was recorded by a wordprocessor printer, and the recorded surface was wiped out with ethanol.The recorded portion had a Macbeth density of 0.23.

EXAMPLE 65

The thermal recording sheet of Example 9 was recorded by a wordprocessor printer, and the recorded surface was wiped out with ethanol.The recorded portion had a Macbeth density of 0.16.

EXAMPLE 66

The thermal recording sheet of Example 18 was recorded by a wordprocessor printer, and the recorded surface was wiped out with ethanol.The recorded portion had a Macbeth density of 0.19.

EXAMPLE 67

The thermal recording sheet of Example 3 was toner recorded by a copier(NP6060/Canon co,. ltd). No change in the ground color was noted.

EXAMPLE 68

The thermal recording sheet of Example 5 was toner recorded by a copier(NP6060/Canon co,. ltd). No change in the ground color was noted.

EXAMPLE 69

The thermal recording sheet of Example 28 was toner recorded by a copier(NP6060/Canon co,. ltd). No change in the ground color was noted.

EXAMPLE 70

The thermal recording sheet of Example 53 was toner recorded by a copier(NP6060/Canon co,. ltd). No change in the ground color was noted.Production of Thermal recording sheet containing the optical absorbent.

EXAMPLES 71-77

As described above, an optical recording sheet was produced using3-N,N-diethylamino-6-methyl-7-anilinofluorane (ODB) as a dye precursor,the dimerized urea compound (or the trimerized urea compound) of thepresent invention as a color developer, and a heat melt (opticalabsorbent A) of a bis-dithiobenzylnickel complex and a sensitizer as anoptical absorbent. (Table 10)

Specifically, 94 parts of 4-biphenyl-9-tolylether was mixed with B partsof bi-dithiobenzylnickel complex, heated to 100° to 150° C. to melt, andthen crushed to obtain an optical absorbent. An optical absorbentdispersion of the following composition was milled by a sand grinder toan average particle diameter of 1 micron.

    ______________________________________                                        (Solution E: optical absorbent dispersion)                                    ______________________________________                                        Optical absorbent      4.0      parts                                         10% Aqueous polyvinylalcohol solution                                                                10.0                                                   Water                  6.0.                                                   ______________________________________                                    

Then, the color developer dispersion (Solution A) used in Examples 1-48,the dye precursor (ODB) dispersion (Solution B) used in Examples 1-48,Solution E (optical absorbent dispersion), and a kaolin clay dispersionwere mixed in the following ratio to obtain a coating color.

    ______________________________________                                        Solution A (color developer dispersion)                                                               36.0    parts                                         Solution B (dye precursor dispersion)                                                                 9.2                                                   Solution E (optical absorbent dispersion)                                                             20.0                                                  Kaolin clay (50% dispersion)                                                                          12.0                                                  ______________________________________                                    

The coating color was coated on one side of a 50 g/m² base paper in anamount of 6.0 g/m² and dried, and the sheet was supercalendered to aflatness of 500 to 600 seconds to obtain an optical recording sheet.

EXAMPLE 78-81

An optical recording sheet was produced using NK-2612 (Nippon KankoShikiso Kenkyusho) (optical absorbent B) as an optical absorbent inplace of the heat melt of bis-dithiobenzylnickel complex and asensitizer. (Table 10)

First, the following aqueous optical absorbent solution was prepared.

    ______________________________________                                               (Solution F: aqueous optical absorbent solution)                       ______________________________________                                               NK-2612                                                                              0.04         parts                                                     Water  3.96.                                                           ______________________________________                                    

Then, the color developer dispersion (Solution A) used in Examples 1-48,the dye precursor (ODB) dispersion (Solution B) used in Examples 1-48,Solution E (optical absorbent dispersion), and a kaolin clay dispersionwere mixed in the following ratio to obtain a coating color.

    ______________________________________                                        Solution A (color developer dispersion)                                                              36.0     parts                                         Solution B (dye precursor dispersion)                                                                9.2                                                    Soluti;on F (optical absorbent solution)                                                             4.0                                                    Kaolin clay (50% dispersion)                                                                         12.0                                                   ______________________________________                                    

The coating color was coated on one side of a 50 g/m² base paper in anamount of 6.0 g/m² and dried, and the sheet was supercalendered to aflatness of 500 to 600 seconds to obtain an optical recording sheet.

EXAMPLES 82-84

An optical recording sheet was produced using toluenedithiolnickelcomplex (optical absorbent C) as an optical absorbent in place of theheat melt of bis-dithiobenzylnickel complex and a sensitizer.

First, an optical absorbent dispersion (Solution G) was milled by a sandgrinder to an average particle diameter of 1 micron.

    ______________________________________                                        Solution G (optical absorbent color developer dispersion)                     ______________________________________                                        Compound A6 (or compound A8 or A66)                                                                   6.0     parts                                         Toluenedithiolnickel complex                                                                          1.0                                                   10% Aqueous polyvinylalcohol solution                                                                 18.8                                                  Water                   10.2.                                                 ______________________________________                                    

Then, the optical absorbent color developer dispersion (Solution G), thedye precursor (ODB) dispersion (Solution B) used in Examples 1 -48, anda kaolin clay dispersion were mixed in the following ratio to obtain acoating color.

    ______________________________________                                        Solution G (optical absorbent color                                                                  36.0     parts                                         developer dispersion)                                                         Solution B (dye precursor dispersion)                                                                9.2                                                    Kaolin clay (50% dispersion)                                                                         12.0                                                   ______________________________________                                    

The coating color was coated on one side of a 50 g/m² base paper in anamount of 6.0 g/m² and dried, and the sheet was supercalendered to aflatness of 500 to 600 seconds to obtain an optical recording sheet.

EXAMPLES 85-86

Using ODB-2 or NEW-Blue as a dye precursor in place of ODB, the sameprocedure as in Examples 7114 77 was used to obtain an optical recordingsheet.

EXAMPLES 87-88

Using ODB-2 or NEW-Blue as a dye precursor in place of ODB, the sameprocedure as in Examples 78-81 was used to obtain an optical recordingsheet.

EXAMPLES 89-90

Using NEW-Blue or I-red as a dye precursor in place of ODB, the sameprocedure as in Examples 82-84 was used to obtain an optical recordingsheet. Evaluation of optical recording sheets

The resulting optical recording sheets were subjected to a recordabilitytest. Recordability test (optical recording):

Using a laser plotter described in Japanese OPI 3-239598, the opticalrecording sheet was irradiated with a laser light, and the recordedportion was measured by a Macbeth densitometer. Using a 30 mWsemiconductor laser LT015MD (Sharp) with an oscillation wavelength of830 nm as a recording light sources, an aspheric plastic lens AP4545(Konica) with a numerical aperture of 0.45 and a focal length of 4.5 mmas an optical converging lens, a recording speed of 50 mm/sec, and arecording interval of 50 microns, a 1 cm square overall recording wasobtained. The evaluation results are shown in Table 10.

                  TABLE 10                                                        ______________________________________                                                Color                Optical  Recorda-                                Entry   developer   Dye      absorbent                                                                              bility test                             ______________________________________                                        Example 71                                                                            Compound A3 ODB      Absorbent A                                                                            1.38                                    Example 72                                                                            Compound A6 ODB      Absorbent A                                                                            1.28                                    Example 73                                                                            Compound A8 ODB      Absorbent A                                                                            1.25                                    Example 74                                                                            Compound A19                                                                              ODB      Absorbent A                                                                            1.40                                    Example 75                                                                            Compound A39                                                                              ODB      Absorbent A                                                                            1.00                                    Example 76                                                                            Compound A49                                                                              ODB      Absorbent A                                                                            1.22                                    Example 77                                                                            Compound A68                                                                              ODB      Absorbent A                                                                            1.02                                    Example 78                                                                            Compound A6 ODB      Absorbent B                                                                            1.26                                    Example 79                                                                            Compound A7 ODB      Absorbent B                                                                            1.33                                    Example 80                                                                            Compound A16                                                                              ODB      Absorbent B                                                                            1.10                                    Example 81                                                                            Compound A63                                                                              ODB      Absorbent B                                                                            1.04                                    Example 82                                                                            Compound A6 ODB      Absorbent C                                                                            1.27                                    Example 83                                                                            Compound A8 ODB      Absorbent C                                                                            1.26                                    Example 84                                                                            Compound A66                                                                              ODB      Absorbent C                                                                            1.01                                    Example 85                                                                            Compound A6 ODB      Absorbent A                                                                            1.27                                    Example 86                                                                            Compound A6 ODB      Absorbent A                                                                            1.02                                    Example 87                                                                            Compound A6 ODB      Absorbent B                                                                            1.26                                    Example 88                                                                            Compound A6 ODB      Absorbent B                                                                            1.00                                    Example 89                                                                            Compound A6 ODB      Absorbent C                                                                            1.01                                    Example 90                                                                            Compound A8 ODB      Absorbent C                                                                            0.98                                    ______________________________________                                         Absorbent A: bisdithiobenzylnickel complex/sensitizer                         Absorbent B: NK2612                                                           Absorbent C: toluenedithiolnickel complex                                

As described above, it can be seen that the dimerized or trimerized ureacompound of the present invention is an epoch-making color developerthat can provide a recording with a practical image density by a thermalhead or the like while without a substantial change in ground color atenvironmental temperatures of 120° to 150° C. Therefore, the presentinvention has the following effects.

(1) A thermal recording sheet superior in storage stability such as heatresistance and solvent resistance compared to conventional thermalrecording sheets.

(2) The thermal recording sheet can be used under severe conditions(e.g. at temperatures of 90°-150° C.) at which conventional productscould not be used.

(3) Since the thermal recording sheet does not undergo discolorationwhen written with an oil ink, it can be freely written using thesewriting means.

(4) The thermal recording sheet can be simply heat laminated by a simplelaminator or the like. Cards and the like can also be easily prepared.

(5) The thermal recording sheet can be toner recorded since the groundcolor is stable even when passed through a heat roll.

Further, the thermal recording sheet having an erasing function has thefollowing advantages:

(6) A new recording system, which enables repeated color recording anderasing, which leads to resource-saving.

(7) Some types of the sheet can be erased merely by passing through aheat roll, without strict temperature control.

(8) Unlike liquid crystals, the thermal recording sheet can be used asimple display means that can be recorded and erased using differentthermal energies.

The thermal recording sheet of the present invention can also beincorporated with an optical absorbent for use as an optical recordingsheet having similar effects.

We claim:
 1. A thermal recording sheet having a thermal recording layercontaining a colorless or pale colored dye precursor and a colordeveloper reactable with said dye precursor upon heating to develop acolor, wherein said color developer is a urea compound having at leasttwo groups of Formula (1) in the molecule, and the thermal recordinglayer contains at least one of said urea compound: ##STR14## wherein Xis alkyl group having 1 to 12 carbon atoms, halogenated alkyl grouphaving 1 to 6 carbon atoms, aralkyl group having 7 to 14 carbon atoms,alkoxy group having 1 to 12 carbon atoms, aryloxy group having 6 to 12carbon atoms, alkoxycarbonyl group having 1 to 12 carbon atoms, acylgroup having 1 to 12 carbon atoms, dialkylamino group having 1 to 12carbon atoms, arylalkylamino group having 7 to 12 carbon atoms,arylamino group having 6 to 12 carbon atoms, acylamino group having 1 to12 carbon atoms, nitro group, cyano group, halogen group or hydrogengroup.
 2. A thermal recording sheet having a thermal recording layercontaining a colorless or pale colored dye precursor and a colordeveloper reactable with said dye precursor upon heating to develop acolor, wherein said color developer is a urea compound of Formula (2),(3), or (4), and the thermal recording layer contains at least one ofsaid urea compounds: ##STR15## wherein X is alkyl group having 1 to 12carbon atoms, halogenated alkyl group having 1 to 6 carbon atoms,aralkyl group having 7 to 14 carbon atoms, alkoxy group having 1 to 12carbon atoms, aryloxy group having 6 to 12 carbon atoms, alkoxycarbonylgroup having 1 to 12 carbon atoms, acyl group having 1 to 12 carbonatoms, dialkylamino group having 1 to 12 carbon atoms, arylalkylaminogroup having 7 to 12 carbon atoms, arylamino group having 6 to 12 carbonatoms, acylamino group having 1 to 12 carbon atoms, nitro group, cyanogroup, halogen group, or hydrogen group, A denotes a divalent groupcomprising 30 or less carbon atoms, and B denotes a trivalent groupcomprising 30 or less carbon atoms, n is an integer from 1 to
 3. 3. Thethermal recording sheet of claim 2, wherein the divalent group (A) ofsaid color developer of Formula (3) is an alkylene group having 1 to 12carbon atoms and having a branched chain.
 4. The thermal recording sheetof claim 2, wherein the divalent group (A) of said color developer ofFormula (3) is a plurality of alkylenes group having 1 to 12 carbonatoms linked with nitrogen or oxygen.
 5. The thermal recording sheet ofclaim 2, wherein the divalent group (A) of said color developer ofFormula (3) is a group containing at least one substituted orunsubstituted cycloalkyl group having 1 to 20 carbon atoms.
 6. Thethermal recording sheet of claim 2, wherein the divalent group (A) ofsaid color developer of Formula (3) is a group having one substituted orunsubstituted aromatic ring and having 1 to 20 carbon atoms.
 7. Thethermal recording sheet of claim 2, wherein the divalent group (A) ofsaid color developer of Formula (3) is a group having at least twosubstituted or unsubstituted aromatic rings and having 1 to 30 carbonatoms.
 8. The thermal recording sheet of claim 2, wherein the divalentgroup (A) of said color developer of Formula (3) is a group containing asubstituted or unsubstituted piperazine ring and having 4 to 12 carbonatoms.
 9. A thermal recording sheet having a thermal recording layercontaining a colorless or pale colored dye precursor and a colordeveloper reactable with said dye precursor upon heating to develop acolor, wherein said color developer is a urea compound of Formula (5),and the thermal recording layer contains at least one of said ureacompound: ##STR16## wherein X is alkyl group having 1 to 12 carbonatoms, halogenated alkyl group having 1 to 6 carbon atoms, aralkyl grouphaving 7 to 14 carbon atoms, alkoxy group having 1 to 12 carbon atoms,aryloxy group having 6 to 12 carbon atoms, alkoxycarbonyl group having 1to 12 carbon atoms, acyl group having 1 to 12 carbon atoms, dialkylaminogroup having 1 to 12 carbon atoms, arylalkylamino group having 7 to 12carbon atoms, arylamino group having 6 to 12 carbon atoms, acylaminogroup having 1 to 12 carbon atoms, nitro group, cyano group, halogengroup, or hydrogen group, n is an integer from 1 to 3, and m is aninteger from 1 to
 12. 10. A thermal recording card comprising thethermal recording sheet of claim 2 or
 9. 11. A reversible recordingmethod comprising the steps of: recording on the thermal recording sheetof claim 2 or 9 erasing the recorded image by applying heat to therecorded portion, and again thermally recording.
 12. A reversiblerecording method comprising the steps of: recording on the thermalrecording sheet of claim 2 or 9, contacting the recorded portion with analcoholic solvent to erase the recorded image, and again thermallyrecording.
 13. A thermal recording sheet having a thermal recordinglayer containing a colorless or pale colored dye precursor and a colordeveloper reactable with said dye precursor upon heating to develop acolor, wherein said color developer is a urea compound of Formula (2),and the thermal recording layer contains at least one of said ureacompound: ##STR17## wherein X is alkyl group having 1 to 12 carbonatoms, halogenated alkyl group having 1 to 6 carbon atoms, aralkyl grouphaving 7 to 14 carbon atoms, alkoxy group having 1 to 12 carbon atoms,aryloxy group having 6 to 12 carbon atoms, alkoxycarbonyl group having 1to 12 carbon atoms, acyl group having 1 to 12 carbon atoms, dialkylaminogroup having 1 to 12 carbon atoms, arylalkylamino group having 7 to 12carbon atoms, arylamino group having 6 to 12 carbon atoms, acylaminogroup having 1 to 12 carbon atoms, nitro group, cyano group, halogengroup, or hydrogen group.
 14. A thermal recording sheet having a thermalrecording layer containing a colorless or pale colored dye precursor anda color developer reactable with said dye precursor upon heating todevelop a color, wherein said color developer is a urea compound ofFormula (3), and the thermal recording layer contains at least one ofsaid urea compound: ##STR18## wherein X is alkyl group having 1 to 12carbon atoms, halogenated alkyl group having 1 to 6 carbon atoms,aralkyl group having 7 to 14 carbon atoms, alkoxy group having 1 to 12carbon atoms, aryloxy group having 6 to 12 carbon atoms, alkoxycarbonylgroup having 1 to 12 carbon atoms, acyl group having 1 to 12 carbonatoms, dialkylamino group having 1 to 12 carbon atoms, arylalkylaminogroup having 7 to 12 carbon atoms, arylamino group having 6 to 12 carbonatoms, acylamino group having 1 to 12 carbon atoms, nitro group, cyanogroup, halogen group, or hydrogen group, A denotes a divalent groupcomprising 30 or less carbon atoms, and n is an integer from 1 to
 3. 15.A thermal recording sheet having a thermal recording layer containing acolorless or pale colored dye precursor and a color developer reactablewith said dye precursor upon heating to develop a color, wherein saidcolor developer is a urea compound of Formula (4), and the thermalrecording layer contains at least one of said urea compound: ##STR19##wherein X is alkyl group having 1 to 12 carbon atoms, halogenated alkylgroup having 1 to 6 carbon atoms, aralkyl group having 7 to 14 carbonatoms, alkoxy group having 1 to 12 carbon atoms, aryloxy group having 6to 12 carbon atoms, alkoxycarbonyl group having 1 to 12 carbon atoms,acyl group having 1 to 12 carbon atoms, dialkylamino group having 1 to12 carbon atoms, arylalkylamino group having 7 to 12 carbon atoms,arylamino group having 6 to 12 carbon atoms, acylamino group having 1 to12 carbon atoms, nitro group, cyano group, halogen group, or hydrogengroup, B denotes a trivalent group comprising 30 or less carbon atoms,and n is an integer from 1 to 3.